Why do i get a lightheaded feeling or like im going to pass out whenever i get the sensation of needing to poop?

Straining to have a bowel movement can stimulate the vagus nerve, which controls involuntary body functions such as heart rate, blood pressure, or breathing. If the vagus nerve is overstimulated this can cause a vasovagal response in some people leading to passing out, lightheadedness, dizziness, nausea, etc. Drink plenty of water, increase your fiber intake, & stay active to prevent constipation.